Abrupt &#34;delta-like&#34; doping in Si and SiGe films by UHV-CVD

ABSTRACT

A structure and method of forming an abrupt doping profile is described incorporating a substrate, a first epitaxial layer of Ge less than the critical thickness having a P or As concentration greater than 5×10 19  atoms/cc, and a second epitaxial layer having a change in concentration in its first 40 Å from the first layer of greater than 1×10 19  P atoms/cc. Alternatively, a layer of SiGe having a Ge content greater than 0.5 may be selectively amorphized and recrystalized with respect to other layers in a layered structure. The invention overcomes the problem of forming abrupt phosphorus profiles in Si and SiGe layers or films in semiconductor structures such as CMOS, MODFET&#39;S, and HBT&#39;s.

FIELD OF THE INVENTION

[0001] This invention relates to semiconductor films with steep dopingprofiles and more particularly to forming abrupt “delta-like” doping inthin layers from 5-20 nm thick suitable for Si or SiGe CMOS,modulation-doped field-effect transistors (MODFET's) devices, andheterojunction bipolar transistors (HBT's) using in-situ doping in aultra high vacuum-chemical vapor deposition (UHV-CVD) reactor.

BACKGROUND OF THE INVENTION

[0002] In-situ phosphorus doping in epitaxial Si and SiGe films orlayers using PH₃ has been known to demonstrate a very slow incorporationrate of P due to the “poisoning effect” of phosphine on the Si(100)surface. An example of such a doping behavior is shown in FIG. 1 bycurve 11. Curve portion 13-14 of curve 11 shows the slow “transient”trailing edge observed in the SIMS profile and corresponds to the slowincorporation rate of P into the silicon film. In FIG. 1 the ordinaterepresents P concentration in atoms/cc and the abscissa represents depthin angstroms.

[0003] The incorporation of P into a Si layer is increased by theaddition of a Ge containing gas (7%) along with phosphine in thereaction zone of a UHV-CVD reactor and has been described in U.S. Pat.No. 5,316,958 which issued May 31, 1994 to B. S. Meyerson and assignedto the assignee herein. The phosphorus dopant was incorporated duringUHV-CVD in the proper substitutional sites in the silicon lattice asfully electrically active dopants. The amounts of Ge used were smallenough that the primary band gap reduction mechanism is the presence ofthe n-type dopants at relatively high levels instead of the effect ofthe Ge. In '958, FIG. 2 shows phosphorus being incorporated into a Silayer during UHV-CVD with and without the addition of 7% Ge containinggas. With 7% Ge containing gas, a decade increase in P concentrationwould be incorporated in 250 to 500 Å into a silicon layer as shown, forexample, by the rate of incorporation from 7×10¹⁸ atoms/cc to 5×10¹⁹atoms/cc in FIG. 2 of '958.

[0004] Another well known problem associated with in-situ phosphorus orboron doping in silicon CVD is its “memory effect” as shown by curveportion 15-16 in FIG. 1 for the case of phosphorus herein which tends tocreate an undesirable high level of dopant in the background due to its“autodoping behavior”. This “memory effect” is also evident in the SIMSanalysis shown in FIG. 1. The “memory effect” corresponds to a very slowfall or decrease in the phosphorus concentration which stems from aresidual background autodoping effect. Hence, in-situ doping typicallygenerates a very undesirable “smearing out” of the dopant profile insilicon films formed by CVD.

[0005]FIG. 2 shows curve 11 which is the same as shown FIG. 1 and whichillustrates the doping profile of the prior art using PH₃. Curve 20shows a desired or targeted profile having a width of 100 angstroms. InFIG. 2, the ordinate represents P concentration in atoms/cc and theabscissa represents depth in angstroms. Curve 11 has a dopant profile ofat least 5 times wider or thicker than the targeted profile of 100Angstroms in width or in depth as shown by curve 20.

[0006] As device dimensions shrink and especially for futurecomplementary metal oxide semiconductor (CMOS) logic, MODFET's, andHBT's incorporating SiGe layers, very thin layer structures having awidth or thickness of 5-20 nm of high doping P concentrations will beneeded which are impossible to obtain with present technology at thispoint using present ultra high vacuum-chemical vapor deposition(UHV-CVD) or standard silicon CVD processing.

SUMMARY OF THE INVENTION

[0007] In accordance with the present invention, a structure is providedhaving an increasing or decreasing abrupt doping profile comprising asubstrate such as Si or SiGe having an upper surface, a first epitaxiallayer of substantially Ge formed over the upper surface, the first layerhaving a thickness in the range from 0.5 to 2 nm and doped e.g. withphosphorus or arsenic to a level of about 5×10¹⁹ atoms/cc, and a secondepitaxial layer of a semiconductor material having any desiredconcentration of dopants. The second layer may be Si or Si_(1-x)Ge_(x).The concentration profile from the edge or upper surface of the firstlayer to 40 Å into the second layer may change by greater than 1×10¹⁹dopant atoms/cc.

[0008] The invention further provides a method comprising the steps ofselecting a substrate having an upper surface, growing a first epitaxiallayer of substantially Ge thereover less than its critical thickness anddoped with phosphorus to a level of about 5×10¹⁹ atoms/cc, growing asecond epitaxial layer selected from the group consisting of Si andSiGe, the second epitaxial layer having any desired doping profile. Thepresence of the epitaxial Ge layer accelerates the incorporation rate ofthe P or As doping into the Ge layer, thereby eliminating the slowtransient behavior. The initial, in-situ doping level is determined bythe dopant flow in SCCM of the PH₃/He mixture. The final overall dopingprofile may be controlled as a function of 1/GR where GR is the growthrate of the first and second layer. The dopant may be supplied orcarried by phosphine (PH₃) or Tertiary Butyl Phosphine (TBP) gas in thecase of P and AsH₃ or Tertiary Butyl Arsine (TBA) in the case of As in aUHV-CVD reactor.

[0009] To eliminate background “autodoping effect”, the structure withphosphorus doping as shown in FIG. 3 is transferred to a load chamber orload lock, while the growth chamber is purged of the backgroundphosphorus. This growth/interrupt/growth process involves hydrogenflushing of the UHV-CVD reactor during interrupt. Then, a coating of Sior SiGe is grown on the sidewalls and/or heated surfaces of the UHV-CVDreactor at high temperature to isolate, eliminate or cover the residualphosphorus atoms prior to reintroducing the structure for furtherdeposition. Alternatively, a second growth chanber i.e. UHV-CVD reactorcoupled to the load chamber may be used where further undoped layers maybe deposited with very low levels of phosphorus.

[0010] A second epitaxial layer 40 and/or a third epitaxial layer 44 ofSi or SiGe shown in FIG. 3 may now be grown with a background dopingprofile that drops or decreases to less than 5×10¹⁶ atoms/cc after a 300Å film is grown over layer 36 of structure 30 shown in FIG. 3.

[0011] The invention further provides a method for forming abrupt dopingcomprising the steps of forming a layered structure of semiconductormaterial, selectively amorphizing a first layer having a high Ge contentgreater than 0.5, and crystallizing the amorphized first layer by solidphase regrowth. The amorphized first layer may be formed by ionimplantation.

[0012] The invention further provides a field effect transistorcomprising a single crystal substrate having source and drain regionswith a channel therebetween and a gate electrode above the channel tocontrol charge in said channel and a first layer of Ge less than thecritical thickness doped with a dopant of phosphorus or arsenicpositioned below the channel and extending through the source and drainregions.

[0013] The invention further provides a field effect transistorcomprising a single crystal substrate, a first layer of Ge less than thecritical thickness formed on the substrate and doped with a dopant ofphosphorus or arsenic, a second layer of undoped SiGe epitaxially formedon the first layer, a third layer of strained undoped simiconductormaterial of Si or SiGe, a source region and a drain region with achannel therebetween and a gate electrode above the channel to controlcharge in the channel.

[0014] The invention further provides a field effect transistorcomprising a single crystal substrate, an oxide layer formed on thesubstrate having an opening, a gate dielectric and a gate electrodeformed in the opening over the substrate, a source and drain regionformed in the substrate aligned with respect to the gate electrode, adielectric sidewall spacer formed on either side of the gate electrodeand above a portion of the source and drain regions, a first layer of Geless than the critical thickness doped with a dopant of phosphorus orarsenic selectivel position over exposed portions of the source anddrain regions, a second layer of semiconductor material selected fromthe group consisting of Si and SiGe doped with a dopant of phosphorus orarsenic epitaxially formed over the first layer to form raised sourceand drain regions.

BRIEF DESCRIPTION OF THE DRAWING

[0015] These and other features, objects, and advantages of the presentinvention will become apparent upon consideration of the followingdetailed description of the invention when read in conjunction with thedrawing in which:

[0016]FIG. 1 is a graph of P concentration versus depth in a SiGesubstrate showing an actual concentration profile of the prior art.

[0017]FIG. 2 is a graph of P concentration versus depth in a SiGesubstrate showing an actual concentration profile to a desired profile.

[0018]FIG. 3 is a cross section view of a first embodiment of theinvention.

[0019]FIG. 4 is a graph of P dopant concentration versus depth and of Gein Si_(1-x)Ge_(x) versus depth illustrating the invention.

[0020]FIG. 5 is a graph of P concentration versus PH₃/He mixture flowrate in SCCM.

[0021]FIG. 6 is a graph of measured conductance versus depth as layersare removed and the projected P concentration versus depth in the layer.

[0022]FIG. 7 is a cross section view of a layered structure.

[0023]FIG. 8 is a cross section view of a layered structure having anamorphized layer.

[0024]FIG. 9 is a cross section view of a second embodiment of theinvention.

[0025]FIG. 10 is a cross section view showing an intermediate step informing the embodiment of FIG. 11.

[0026]FIG. 11 is a cross section view showing a third embodiment of theinvention.

[0027]FIG. 12 is a cross section view showing a fourth embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Referring to the drawing and in particular to FIG. 3, a crosssection view of structure 30 having an abrupt phosphorus or arsenicprofile or abrupt layer doping (ALD) is shown. A substrate 32 having anupper surface 33 may be for example single crystal Si or SiGe. A firstlayer 36 of 100% or substantially Ge is epitaxially formed on uppersurface 33 having a thickness less than the critical thickness and maybe, for example, 0.5 to 2 nm and is doped with P or As.

[0029] The effect of the thickness of first layer 36 is not to increasethe doping concentration of P or As, but the effect is to increase thesheet dose, which is the doping concentration multiplied by the dopedlayer thickness. The doping concentration is controlled by the flow rateof the dopant source gas and by the growth rate of first layer 36, whichin turn, is controlled by the flow rate of the Ge source gas which maybe, for example, GeH₄.

[0030] The critical thickness of a layer is the thickness after whichthe layer relaxes to relieve strain due to lattice mismatch which for aGe layer is about 1.04 the lattice spacing of a Si layer. Normally, themechanism for relieving strain is the generation of crystal latticedefects e.g. misfit dislocations which may propagate to the surface inthe form of threading dislocations. A relaxed layer is no longer latticematched to the layer below.

[0031] First layer 36 is substantially Ge and may be 100% Ge. A secondlayer 40 comprising Si or SiGe doped to any desired level is formed overfirst layer 36. Second layer 40 may be formed in a UHV-CVD reactor witha dopant source gas such as PH₃. A Si source gas such as SiH₄ or Si₂H₆and a Ge source gas such as GeH₄ may be used. A third layer 44comprising doped or undoped Si or SiGe may be formed in a UHV-CVDreactor over second layer 40.

[0032] A UHV-CVD. reactor suitable for use in depositing first layer 36,second layer 40 and third layer 44 is available from Leybold-HeraeusCo., Germany and is described in U.S. Pat. No. 5,181,964 which issuedJan. 26, 1993 to B. S. Meyerson and in U.S. Pat. No. 5,607,511 whichissued Mar. 4, 1997 to B. S. Meyerson which are incorporated herein byreference. The operation of the reactor and suitable methods fordepositing Si and SiGe films is described in U.S. Pat. No. 5,298,452which issued Mar. 29, 1994 to B. S. Meyerson and which is incorporatedherein by reference.

[0033] Referring to FIG. 4, secondary ion mass spectroscopy (SIMS) datawas obtained from a multilayered structure of Si_(1-x)Ge_(x) doped withphosphorus. In FIG. 4, the ordinate on the right side represents Gerelative intensity with respect to curve 50 and the abscissa representsapproximate depth in microns below the surface of the multilayeredstructure. The structure at a depth of 1.17 μm is 100% Si with theamount of Ge, X equal to zero. AS shown by level curve portions 51-57 oncurve 50, the amount X of Ge is 0.05 at from 1.12 to 1.08 μm, 0.10 atfrom 1.03 to 0.99 μm, at 0.15 from 0.93 to 0.59 μm, 0.20 from 0.52 to0.24 μm, 0.25 from 0.2 to 0.17 μm, 1.0 from 0.17 to 0.13 μm, and 0.25from 0.13 to 0.3 μm, respectively. The layers were epitaxially grownover a single crystal substrate by varying the flow rate of GeH₄. Curve60 shows the in-situ phosphorus doping in the multilayers as a functionof depth using PH₃ as the dopant source gas. In FIG. 4, the ordinate onthe left side represents P concentration (atoms/cc) with respect tocurve 60 and the abscissa represents depth. The 100% seed layer of 0.5-2nm at the depth of 0.17 μm allows for a very abrupt, phoshorus dopingprofile to occur as shown by curve 60 and particularly at curve portion62-63, in FIG. 4 and at the same time allows for high doping Pconcentrations to be achieve controllably as shown by curve 70 in FIG.5.

[0034]FIG. 5 is a graph of the phosphorus concentration (atoms/cc)versus 100 PPM PH₃/He mixture flow (SCCM). In FIG. 5, the ordinaterepresents phosphorus concentration (atoms/cc) and the abscissarepresents flow (SCCM).

[0035] Due to the limitation of the SIMS technique to resolve very thinlayers, the SIMS result shown in FIG. 4 gives a dopant profile width ofabout 150-200 Å at full width half maximum (FWHM). To better resolve thedopant profile, Hall measurements were used to measure and profile theactive carriers throughout the doped sample by stepwise etching throughthe entire doped structure coupled with direct Hall measurement aftereach etching step.

[0036]FIG. 6 is a graph showing the conductance versus depth and showingthe phosphorus concentration versus depth in a multilayered structureusing direct Hall measurements. In FIG. 6 the ordinate on the left siderepresents conductance (mS) and the abscissa represents depth below thesurface of a multilayered Si_(1-x)Ge_(x) structure having a layer of 1-2nm Ge at a depth of 115 nm. Curve 80 shows the conductance as measuredversus depth. The conductance increases from 0 at 120 nm to 0.21 at 110nm. The dopant profile as measured by the electrical measurement isshown by curve 88. Curve 80 and/or its data points were used to generatecurve 88 shown in FIG. 6 which shows the actual phosphorus dopingprofile. Curve 88 was generated by dividing the carrier density asdetermined from the conductance shown by curve 80 at the respectiveetched depth by the etch layer thickness. In FIG. 6, the ordinate on theright side represents P concentration (atoms/cc). Curve 86 shows theprojected concentration based on curve 88 which shows the peakconcentration rising abruptly from less than 1×10¹⁵ at 121 nm to 5×10¹⁹at 115 nm corresponding to a 13 Å per decade rise in P concentration.The FWHM based on curve 86 which itself is projected from curve 88 is 8nm at a peak concentration of 2×10¹⁹ atoms/cc. The doping concentrationas shown by curve 86 decreases from 5×10¹⁹ atoms/cc at 115 nm to about8×10¹⁷ atoms/cc at 109 nm and 1×10¹⁷ atoms/cc at 64.9 nm. The decreasein P concentration from 115 nm to 64.9 nm corresponds to a 20 nm perdecade fall or decrease in P concentration.

[0037] It is noted that PH₃ has a sticking coefficient S of 1.0 whileSiH₄ has a sticking coefficient S of 1×10³ to 1×10⁻⁴. The doping profileof P is a function of 1/GR where GR is the growth rate of the film.

[0038] Further, to eliminate background autodoping when an abruptreduction in the P concentration is desired, a growth interrupt methodis provided. The substrates or wafers are removed from the growthchamber or UHV-CVD to another vacuum chamber such as a load lock ortransfer chamber or another UHV-CVD reactor or furnace where no PH₃ hasbeen flown prior to loading. Then, SiH₄ and GeH₄ gases are flown in thegrowth chamber to coat the walls or heated surfaces of the growthchamber to bury or to isolate the P on the sidewalls. Then, thesubstrates or wafers are introduced or moved back into the main orgrowth chamber and the growth of Si or Si_(1-x)Ge_(x) is continued.Alternatively, another UHV-CVD reactor or furnace coupled to thetransfer chamber may be used to continue the growth of Si or SiGe withreduced or no P or As doping.

[0039] Another method for achieving abrupt P doping, is to grow a firstepitaxial layer 80 in the range from 1 to 10 nm thick of Si_(1-x)Ge_(x)on a substrate 82 as shown in FIG. 7. The higher the value of X thebetter for converting layer 80 to amorphous material by ion implantationby ions 83 shown in FIG. 8; X may be, for example, greater than 0.5.First epitaxial layer 80 may be unstrained or a strained layer due tolattice mismatch with respect to substrate 82. A second epitaxial layer84 may be grown over first epitaxial layer 80. Layer 84 may be Si orSiGe and may be unstrained or strained. Then using ion implantationshown in FIG. 8, the first epitaxial layer 80 may be selectivelyamorphized to form layer 80′ shown in FIG. 8 by ions 83 with respect tolayer 84 and substrate 82 at a dose in the range from about 10¹³ toabout 10¹⁴ atoms/cm² or higher; layer 84 and any other Si or SiGe layerswill not be amorphized. The Ge content of layer 84 and the other layersshould be less than the content X in layer 80.

[0040] The critical dose for amorphization depends on the implantedspecies as well as on the host lattice. For example, boron does notamorphize Si at any dose, but amorphizes Ge at a dose higher than 1×10¹⁴atoms/cm². Asqenic amorphizes Si at a dose of about 5×10¹⁴ atoms/cm²,while Arsenic amorphizes Ge at a dose of 1×10¹³ atoms/cm². Thus if animplant dose below the amorphization threshold in Si but above that inSiGe or Ge is used, then only the SiGe or Ge will be amorphized. Thedossage peak should be adjusted to occur at the depth of the layer to beamorphized, layer 80.

[0041] Substrate 82 and first epitaxial layer 80 is then heated to atemperature in the range from 400° C. to 500° for a period of time suchas from 1 to 5 hours which results in solid phase recrystallization ofthe amorphized layer to form Si_(1-x)Ge_(x) layer 80″ shown in FIG. 9.

[0042] Recrystallization of amorphous layer 80′ is dependent upon thematerial of the layer. Amorphous Ge recrystallizes at a temperature Tgreater than 350° C., while Si recrystallizes at a temperature T greaterthan 500° C. The combination of amorphization threshold dose andrecrystallization temperature difference between Si and Ge is key toprovide recrystallized layers.

[0043] The alloy SiGe recrystallization temperature will be somewhere inbetween Si and Ge, depending on the Ge content. If thicker doped layersare sought, which are above the critical thickness of Ge on Si, thenSiGe with the highest possible Ge content (that will stay strained)should be used. To maximize the sharpness of the doping profile, thelayers surrounding the doped layer should have the lowest possible Gecontent (depending on the design)

[0044] Dopant activation occurs only in layer 80″. Thus the doped layerthickness 80″ is determined by the original epitaxial layer thickness80. Diffusion of P dopants at the recrystallization temperature isnegligible.

[0045] The above method applies to any species and not just to P. Infact getting sharp p-type implants is very much needed in the channelimplant of 0.25 μm PMOS and will be needed more when the gate length isshrunk. B cannot be used for such super retrograde profiles, and hencepeople have resorted to heavy ions such as In. However, the degradationin channel mobility is higher in that case, and the incorporation of Inat levels higher than 5×10¹⁷ atoms/cm³ is almost impossible.

[0046] An n or p channel field effect transistor 91 is shown in FIG. 9utilizing layer 80″. A dielectric layer 85 may be formed on the uppersurface of layer 84 to form a gate dielectric such as silicon dioxide. Agate 86 may be blanket deposited and patterned above dielectric 85 whichmay be polysilicon. Self aligned shallow souce and drain regions 87 and88 may be formed in layer 84 by ion implantation using gate 86 as amask. Sidewall spacers 89 and 90 may be formed on the sidewalls of gate86. Source and drain regions 87′ and 88′ may be formed in layers 80 and84 and substrate 82 using sidewall spacers 89 and 90 as a mask. Source87 and 87′ and drain 88 and 88′ may be of one type material (n or p) andlayer 80″ may be of the opposite type material. Layer 80″ functions toadjust the threshold voltage of the field effect transistor 91, preventshort channnel effects and prevent punch through between source anddrain.

[0047] Referring to FIG. 10, an intermediate step in forming a fieldeffect transistor is shown. A substrate 95 may be relaxed undoped SiGe.A phosphorous-doped Ge layer 96 is formed thereover as described withreference to FIGS. 3 of 9. An undoped SiGe layer 97 is formed over layer96. A strained undoped Si layer 98 may be formed over layer 97. Layer 98is suitable for an electron or hole gas 99 to be present under propervoltage biasing conditions.

[0048] Referring to FIG. 11, field effect transistor 102 is shown. InFIG. 11, like reference numbers are used for functions corresponding tothe apparatus FIG. 10. Source and drain regions 103 and 104 are formedspaced apart through layers 96-98 and into substrate 95. A gatedielectric 105 may be formed over layer 98 in the region between source103 and drain 104. A gate electrode 106 of polysilicon or metal may beblanket deposited and patterned. Alternately, gate dielectric 105 may bedeleted and a gate electrode of metal may form a Schottky barrier withlayer 98.

[0049] Referring to FIG. 12, a cross section view of field effecttransistor 110 is shown with raised source 40′ and drain 40″. In FIG. 12like references are used for functions corresponding to the apparatus ofFIGS. 3 and 9. Substrate 82′ has a layer of field oxide 112 thereoverwith an opening 113 formed therein. In opening 113, a gate dielectric 85is formed on substrate 82′. A gate electrode 86 is formed such as frompolysilicon and a shallow source 87 and drain 88 are formed by, forexample, ion implantation self aligned with respect to gate electrode86. Next, sidewalls 89 and 90 are formed on either side of gate-electrode 86. Next, a layer 36′ is selectively formed epitaxially onshallow source 87 and drain 88 on substrate 82′ which is phosphorous orarsenic doped. Layer 36′ is Ge or substantially Ge and corresponds tolayer 36 in FIG. 3. Above layer 36′, layer 40′ of Si or SiGe isselectively formed epitaxially which is phosphorous or arsenic dopedduring fabrication. Layer 40′ forms source 117 above shallow source 87and forms drain 118 above shallow drain 88. Metal silicide contacts (notshown) may be made to source 117 and drain 118.

[0050] While there has been described and illustrated a structure havingan abrupt doping profile and methods for forming an abrupt profile, itwill be apparent to those skilled in the art that modifications andvariations are possible without deviating from the broad scope of theinvention which shall be limited solely by the scope of the claimsappended hereto.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:
 1. A structure having an abrupt doping profile comprising: a single crystal semiconductor substrate having an upper surface, a first epitaxial layer of Ge over said upper surface, said first epitaxial layer having a thickness less than the critical thickness, said first epitaxial layer having a concentration of dopant greater than 5×10¹⁹ atoms/cc, said dopant selected from the group consisting of phosphorus and arsenic, and a second epitaxial layer of a semiconductor material over said first epitaxial layer.
 2. The structure of claim 1 wherein said second layer comprises a material selected from the group consisting of Si and SiGe.
 3. The structure of claim 1 wherein said first layer has a thickness in the range from 0.5 to 2 nm.
 4. The structure of claim 1 wherein said second layer has a concentration change from said first layer into 40 Å of said second layer of greater than 1×10¹⁹ atoms/cc.
 5. The structure of claim 1 further including a third epitaxial layer of semiconductor material having a doping profile with a dopant concentration less than 5×10¹⁸ atoms/cc.
 6. The structure of claim 1 wherein said second epitaxial layer having a thickness of at least 300 Å and having a doping of P less than 5×10¹⁶ atoms/cc for a predetermined thickness after its initial 300 Å thickness.
 7. A method for forming an abrupt doping profile comprising the steps of: selecting a single crystal semiconductor substrate having a major upper surface, first forming a first epitaxial layer of Ge over said upper surface, said first epitaxial layer having a thickness less than the critical thickness, said step of first forming including the step of incorporating a concentration of dopant greater than 5×10¹⁹ atoms/cc, said dopant selected from the group consisting of phosphorus and arsenic, and second forming a second epitaxial layer of a semiconductor material over said first epitaxial layer.
 8. The method of claim 7 wherein said step of selecting includes the step of selecting a plurality of substrates, each substrate having a major upper surface, and wherein said steps of first forming and second forming are performed with respect to said plurality of substrates.
 9. The method of claim 7 wherein said step of first forming further includes the steps of placing said substrate in a first CVD reactor and flowing a germanium containing gas and a dopant containing gas.
 10. The method of claim 7 wherein said step of first forming further includes the step of adjusting the growth rate of said first epitaxial layer as a function of time.
 11. The method of claim 10 wherein said step of adjusting the growth rate further includes the step of changing the flow rate of said Ge containing gas.
 12. The method of claim 7 wherein said step of first forming further includes the step of terminating said step prior to reaching the critical thickness of said first epitaxial layer.
 13. The method of claim 7 wherein said step of second forming said second epitaxial layer further includes the steps of placing said substrate in a first CVD reactor and flowing a silicon containing gas and a dopant containing gas, said dopant selected from the group consisting of phosphorus and arsenic.
 14. The method of claim 13 wherein said step of second forming further includes the step of adjusting the flow rate of said dopant containing gas as a function of time.
 15. The method of claim 7 wherein said step of forming said second epitaxial layer includes after the step of first forming said first epitaxial layer, the steps of removing said substrate from said first CVD reactor to a load lock having a controlled atmosphere to prevent oxidation of the surface of said second layer, and transferring said substrate to a second CVD reactor having internally exposed surfaces initially free of phosphorus.
 16. The method of claim 7 wherein said step of second forming said second layer includes the steps of removing said substrate from said first CVD reactor to a load lock having a controlled atmosphere to prevent oxidation of the surface, flowing a silicon containing gas into said first CVD having heated surfaces to coat said heated surfaces with a third silicon containing layer to cover said heated surfaces that may contain phosphorus containing layers that may have formed during the formation of said first epitaxial layer, transferring said substrate into said first CVD reactor, and forming a fourth layer on said upper surface of said second epitaxial layer.
 17. The method of claim 16 wherein said step of flowing a silicon containing gas includes the step of flowing the combination of H₂/SiH₄/GeH⁴.
 18. A method for forming abrupt doping within a semiconductor layered structure comprising the steps of: selectively amorphizing a first layer having a high Ge content greater than 0.5, and crystallizing said amorphized first layer by solid phase regrowth.
 19. The method of claim 18 wherein said step of selectively amorphizing includes the step of ion implantation.
 20. The method of claim 18 wherein said step of selectively amorphizing includes first forming second and third layers about said first layer, said second and third layers having a Ge content less than 0.5.
 21. The method of claim 18 wherein said step of selectively amorphizing includes the step of first forming said first layer having a Ge content greater than 0.5.
 22. A field effect transistor comprising: a single crystal substrate having a source region and a drain region with a channel therebetween and a gate electrode above said channel to control charge is said channel and a first layer of Ge less than the critical thickness doped with a dopant selected from the group consisting of phosphorus and arsenic positioned below said channel and extending through said source and drain regions.
 23. The field effect transistor of claim 22 wherein said layer of Ge is in the range from 0.5 to 2 nm thick.
 24. The field effect transistor of claim 22 wherein said channel is in a second epitaxial layer selected from the group consisting of Si and SiGe formed over said first layer.
 25. A field effect transistor comprising: a single crystal substrte, a first layer of Ge less than the critical thickness doped with a dopant selected from the group consisting of phosphorus and arsenic formed on said substrate, a second layer of undoped SiGe epitaxially formed on said first layer, a third layer of strained undoped semiconductor material selected from the group consisting of Si and SiGe, a source region and a drain region with a channel therebetween, and a gate electrode above said channel to control charge in said channel.
 26. The field effect transistor of claim 25 wherein said layer of Ge is in the range from 0.5 to 2 nm thick.
 27. A field effect transistor comprising: a single crystal substrate, an oxide layer formed on said substrate having an opening, a gate dielectric and gate electrode formed in said opening over said substrate, a source and drain region formed in said substrate aligned with respect to said gate electrode, a dielectric sidewall spacer formed on either side of said gate electrode and above a portion of said source and drain regions, a first layer of Ge less than the critical thickness doped with a dopant selected from the group consisting of phosphorus and arsenic selectively positioned over exposed portions of said source and drain regions, a second layer of semiconductor material selected from the group consisting of Si and SiGe doped with a dopant selected from the group consisting of phosphorus and arsenic epitaxially formed over said first layer to form raised source and drain regions.
 28. The field effect transistor of claim 27 wherein said layer of Ge is in the range from 0.5 to 2 nm thick. 